This invention relates to an indirect optical free-space communications system for broadband transmission of high-speed data and a method for broadband transmission of high-speed data.
Communication between computers or other technical systems is important not only in office rooms or computer rooms but increasingly also in the interior of transportation means because data communication is also increasingly important there. For example, larger transportation means, such as buses, trains, airplanes, ships, etc. are equipped with displays, earphone connections, input terminals and/or receivers or data stations, so the passenger can be entertained or informed during the trip and service personnel can exchange data with central data stations, etc. In such applications, very large volumes of data are usually transmitted by a central data memory and/or transmitter to one or more data receivers or vice-versa.
The data transmission, which may be in analog or digital form, is accomplished via electric cable connections between the transmitter and receiver. However, this is associated with a number of disadvantages.
First, cable must be installed to each terminal, i.e., each receiver/transmitter and/or each data station in the interior must be equipped with a suitable plug and the cable must be provided with a suitable plug connection at the location of the connection. The receiver/transmitter therefore has only limited flexible mobility and also needs a standardized plug connection, and the number of possible receivers is predetermined by the number of plugs on the cable or by the number of cables. The location of the data station is also limited by the predetermined spatial arrangement. This is problematical where the number and location of receivers is not predetermined and must be kept flexible. However, equipment with a great many cables and plug connections is expensive and susceptible to trouble.
Electric cables are also exposed to interference due to electromagnetic radiation (EMF). There is a drastic loss of quality during transmission of analog signals in particular but interference also occurs in the case of digital transmission. This problem is especially difficult to handle in transportation means because in a moving vehicle, unlike a fixed building, the environment and the incident radiation cannot be kept constant over the transmission link, so that incident radiation can at most be reduced by overdimensional shielding during travel. However, such highly shielding cables are heavy, expensive and immobile. Furthermore, the transportation means itself may cause a high level of electromagnetic contamination, i.e., EMF.
In addition, an electric cable is not only exposed to incident radiation but also emits radiation itself. This in turn causes EMF with respect to other electronic systems within the transportation means or with respect to other transportation means that happen to be in the vicinity. Furthermore, such a system is also susceptible to interception.
Another disadvantage is the limited data rate which can be transmitted due to the limited bandwidth of electric cables. The higher the data rate, the more difficult is its transmission over an electric connection owing to damping and dispersion and the greater is the problem of EMF and shielding.
To overcome the problem of EMF, an attempt is made to perform the data transmission through optical fibers by means of modulated light signals. A light source is usually amplitude modulated. Frequency and phase modulation are also possible. The light of the transmission source is transmitted to the receiver over a glass or plastic light source conductor that replace the electric cable. The receiver contains a photodetector for converting the optical signal back into an electric signal. Very high data rates can be achieved in this way and EMF is ruled out.
However, inflexibility with regard to the number and location of the stations to be connected in the interior remains a problem, especially since the optical fibers and plug connections cost more in general than the corresponding electric connections and are more difficult to install. Furthermore, it is difficult to provide free plug connections for stations to be connected later and/or it can be done only at great expense.
To solve the problem of cabling, there have been attempts to use wireless communication systems such as those that have long been used in the field of wireless transmission. By analogy with radio and wireless communications, all types of data can be transmitted by wireless transmission. The possible data transmission rate, however, depends on the frequency of the electromagnetic radiation used. Recently there are small microwave transceivers having frequencies in the gigahertz range, which in practice allow data communication in the range of a few Mb/s. However, a disadvantage here is that the data transmission rate is limited in principle, and there is still the problem of EMF radiation and emission of EMF into other systems of the transportation means as well as the problem of security against interception. In the case of transportation means that are used mainly in various countries of the world, such as airplane or ships, the local regulations for the frequency bands and types of modulation that are used must be taken into account, which greatly restricts the choice of available frequency bands and complicates their international use.
An alternative approach is a wireless but optical data transmission system in which the optical radiation modulated with data is transmitted directly into the room and is received by a photodetector (e.g., IrDA standard). This method is used, for example, for data transmission between a mobile computer and a printer. However, the disadvantage of this method is that there must be a direct line-of-sight connection between the transmitter and receiver, and the distance between the two must be relatively small, typically <1-2 meters. The radiation must be within a certain angle range of the receiver, e.g., in the range of less than ±15° and in direct visual contact with it.
IR data transmission systems using scattered IR radiation which have a much greater range and acceptance angle and are suitable for connecting computers in office rooms, for example, are described by F. R. Gfeller, U. Bapst, Proceedings of the IEEE, Vol. 67, No. 11, November 1979.
Another system is described in German Patent DE 101 07 538 B4. Such a system allows a considerable bandwidth in the range of a few Mb/s to approx. 100 Mb/s and ranges of several meters without the requirements of beam alignment due to the use of scattered radiation, where the bandwidth is limited essentially by the multipath propagation which occurs with multiple reflections.
FIG. 1 shows such a known optical free-space communication system for broadband transmission of high-speed data. It includes a transmitter 1, which has a modulable light source and at least one receiver 5a, 5b, 5c, 5d comprising a photodetector to receive light transmitted by the transmitter 1 and converted into an electric signal. The transmitter 1 and receivers 5a, 5b, 5c, 5d are designed so that the light transmitted by the transmitter 1 is detectable and/or detected by the receiver 5a, 5b, 5c, 5d after scattering or reflection on a surface 7 inside a transportation means. The surface 7 is situated inside, i.e., in the interior space of a transportation means, e.g., on a ceiling 8. The light source used may be, for example, one or more LEDs, laser diodes, edge-emitting laser diodes and/or VCSEL lasers, and there may be bidirectional communication between the transmitter and receiver.
A bidirectional design is preferable because of the greater flexibility. Therefore, when speaking of transmitters below, this is understood to refer, for example, in particular to the transmitter or transceiver which is attached to the body of the transportation means, for example, and the term data station is understood to refer, for example, in particular to the receiver or transceiver that is facing the user, i.e., is mounted on the passenger seat and/or on the multimedia unit attached to the seat, on a handheld device or the like. To be able to differentiate this, the following text therefore refers simply to a distributor and a data station.
The object of the present invention is to improve upon the known optical free-space communication system in such a way that the bandwidth is increased. In particular, high data rates in the range of Gbit/s or more on the whole should be transmissable in the interior of transportation means such as airplane or motor vehicles.
This object is achieved by the optical free-space communication system for broadband transmission of high-speed data as claimed and by the method for broadband transmission of high-speed data as claimed. Other advantageous features, aspects and details of the invention are derived from the dependent claims, the description and the drawings.
The inventive indirect optical free-space communication system is used for broadband transmission of high-speed data and includes a transmitter having a modulable light source and a receiver having a photodetector to receive light emitted by the transmitter and converted into an electric signal. The transmitter and receiver are directed at at least one shared surface that reflects light emitted by the transmitter before reaching the receiver. The free-space communication system has a cellular design and includes several cells, each of which comprises at least one transmitter and at least one receiver, and the cells are designed to prevent crosstalk with any of the neighboring cells, so the cells are independent of one another.
High data rates in the range of Gbit/s or even higher can be transmitted optically with this system from one transmitter to multiple receivers (unidirectional) and/or from one transmitter to multiple distributed receivers (unidirectional broadcast) or between transceivers (bidirectional). This system can be used in the interior of transportation means or vehicles, where it allows secure transmission of high data rates at a reduced cost and with increased bandwidth.
With this communication system it is possible to easily transmit data at a high rate in a transportation means, e.g., an automobile, airplane, train, ship, satellite or the like without requiring a direct line-of-sight connection between the transmitter and receiver and without requiring cabling between the transmitter and receiver for data transmission. This invention also overcomes the problem of the EMF and ensures an increased security against interception.
Through this invention, multiple transmitters with respective receivers, e.g., in an airplane having two or more aisles between the rows of seats can utilize the full bandwidth of approximately 100 Mbit/s, for example, independently of one another; this corresponds to doubling the total useable bandwidth. Two or more “cells” of a communication system based on the same physical medium can thus be implemented, and the system does not have any crosstalk due to the relatively good alignment of the optical signal. This is possible only in the optical range used here because such a sharp delineation would not be possible with radio waves because of the non-directional propagation at lower frequencies.
The surface to which the distributors, i.e., transmitters and data stations, i.e., receivers are directed is advantageously situated in the interior space of a transportation means in which the data transmission takes place, preferably in an automobile, airplane, train, ship or satellite. In this way the data transmission can take place in mobile systems in a particularly simple manner. The cost is reduced and comfort is increased because there are no restrictions or at least there are only minor restrictions with regard to the reception site.
Means for beam shaping of the respective transmitter and/or means for limiting the reception opening angle of the respective receiver are advantageously provided to prevent crosstalk. Therefore the optical beam propagation is shaped in a suitable way so as to yield only spatially limited signal propagation. For example this also takes into account the fact that the data rates are limited to approximately 100 Mbit/s due to multipath propagation.
The indirect optical free-space communication system is preferably designed so that the reflection or scattering ranges of the shared surfaces do not overlap mutually within neighboring cells. Therefore independent communication cells can be achieved, e.g., along an aisle in an airplane or in transportation means in general.
For example, in an airplane with two aisles, the ceiling panels along the two aisles may reflect signals from separate transmitters or transceivers, so that there is no overlapping of the surfaces reflecting signals of the transmitters and receivers of the two aisles where said surfaces are reflecting signals, i.e., within the line of sight vision of the receivers.
For example, areas of an airplane along its aisle may reflect the optical signals separately, so that the neighboring shared surfaces of different distributors and data stations of an area do not have any overlap with those of the other area and can communicate independent of one another so that different, independent communication cells that do not have any crosstalk are formed along an aisle. The surfaces reflecting optical signals can be delineated either through a suitable choice of the transmission and reception lenses or by fade-in using optically nontransmitting beam apertures at the border of the mutually independent surfaces reflecting signals.
Therefore one or more shading elements is also preferably provided, serving to shield the light reflected by the shared surface of one cell with respect to a receiver belonging to a different cell.
The light sources of neighboring cells especially advantageously have different wavelengths. In this way the surfaces reflecting signals and/or the surfaces in the field of vision of the receivers can easily overlap at the interfaces, for example. The different wavelengths of the light are 810 nm and 960 nm; these wavelengths are especially easy to separate by simple filters of the receiver. Therefore, corresponding light sources, preferably LEDs or lasers of the two wavelength ranges, are used at the transmission end.
The different measures are especially preferably combined, which yields special advantages.
Although the two aisles of an airplane, for example, are separated by the beam guidance as independent communication cells because of the shadowing that occurs in the borderline area due to the baggage receptacles in the middle area, slightly overlapping cells with different wavelengths are implemented along the aisles. The full reflection/adjustment of the field of vision is accomplished preferably in such a way that although overlapping may be possible (but need not be possible) in the borderline area of neighboring cells, there is no overlapping with the next-but-one cell. Therefore, only two different labelings L1 and L2 are needed along an aisle, alternating approximately in the arrangement L1-L2-L1-L2-L1-L2. This reduces the equipment complexity due to the wavelengths required to a number limited to two, especially since filtering need not be very sharp, but on the other hand, almost any data speed can be achieved due to the alternating use.
This means that the cells are preferably arranged in a row, with the wavelengths of the light alternating in neighboring cells.
However, the cells are advantageously also separated from one another spatially. The area of a cell is a surface of a wall or an object in the interior of a space in which data is transmitted.
The light source of the respective transmitter includes, for example, one or more LEDs, laser diodes, edge-emitting laser diodes and/or VCSEL lasers.
The respective receiver may include a light source and the respective transmitter may include a photodetector to perform bidirectional communication. During operation a single transmitter of a cell may supply a plurality of receivers with identical or different data at the same time.
According to another aspect of the invention, a method for indirect optical transmission of high-speed data is made available. In the method light is generated by means of a light source, and is modulated at the frequency of the data to be transmitted. The modulated light is transmitted onto a surface that reflects and/or scatters the light and the light scattered and/or reflected by the surface is received so that it travels from the light source to the receiver by an indirect route. The modulated light is connected into an electric signal, whereby the transmission and reception of the light take place in multiple independent cells. Each cell includes at least one transmitter and at least one receiver, and crosstalk with a neighboring cell is actively prevented.
The advantages achieved above with respect to the inventive system are also applicable for the inventive method and vice-versa.
The light emitted by the transmitter of a cell is limited in its beam angles such that it does not reach receivers of other cells.
For example, the reception beam angle of the respective receiver of a cell may also be limited in such a way that the light from the light sources of neighboring cells does not reach the receiver.
In particular, the light reflected from the surface of a cell may also be shaded with respect to a receiver belonging to another cell.
Light of different wavelengths is advantageously used in neighboring cells.